There are a number of precipitation strengthened nickel-base superalloys which, because of their strength at high temperatures, are used as materials in fabricating components for use in high temperature sections of gas turbines. The precipitate involved is an intermetallic compound, generally referred to as gamma prime, having the generic formula (Ni.sub.3 (Al, Ti). Alloys hardened by such precipitates are referred to as gamma prime strengthened superalloys. In recent years, while characteristics of such alloys at lower temperatures have not altogether been ignored, the greater emphasis in the development of improved alloys has been centered around performance at high temperatures. High temperature performance has been of concern because of the fact that in new engine designs, as .[..[.gar.]..]. .Badd.gas .Baddend.turbine operating temperatures are increasing to meet the demands for higher efficiency and power output. High temperature performance properties of particular concern include stress-rupture and creep strength, resistance to thermal fatigue, and corrosion resistance.
It is known that thermal fatigue properties are associated with intermediate temperature (1,300.degree. F. - 1,500.degree. F.) ductility. The greater the ductility in this range, the more resistant the alloy is to thermal fatigue. As a general rule, alloys with high temperature rupture and creep strength have inadequate thermal fatigue and hot corrosion resistance. Conversely, alloys with good hot corrosion resistance show poor high temperature rupture, creep and thermal fatigue properties.
While much work has been done in the development of precipitation strengthened high temperature superalloys, no alloy has been found to be entirely satisfactory with respect to fulfilling the strength, ductility and thermal fatigue requirements needed in gas turbine components. Indeed, in recent superalloy developments, strength improvements obtained through composition modification generally have resulted in reduced ductility. In the same manner, alloys designed for improved ductility or toughness or hot corrosion resistance generally possess inadequate strength.
Superalloys suitable for fabricating gas turbine components desirably possess good creep-rupture strength, i.e., resist excessive creep or rupture for long periods of time while under stress at high temperatures. Such alloys also desirably possess good creep-rupture ductility, i.e., deform uniformly and predictably while under stress at high temperature, rather than crack and fracture. Alloys that lack ductility will tolerate little deformation before the onset of crack nucleation, rapid crack propagation, and failure. Use of a material lacking adequate ductility can result in unpredictable and catastrophic engine component failure. A characteristic peculiar to the gamma prime strengthened superalloys is that they are subject to a sharp decrease in creep-rupture ductility and tensile strength at temperatures between about 1,300.degree. F. and 1,500.degree. F. The decrease in ductility is commonly referred to as the "ductility trough," as ductility is higher at temperatures below 1,300.degree. F. and above 1,500.degree. F. It generally has been observed that the higher the strength of an alloy, the more pronounced will be the ductility decrease within the "ductility trough" temperature range. An example would be MAR-M200 (U.S. Pat. No. 3,164,465). This alloy possesses adequate strength for most advanced gas turbine engine requirements, but lack of 1,400.degree. F. ductility in the conventionally cast material precludes its usefulness for turbine components.
To circumvent the low ductility problem while retaining usable high temperature strength, the art, in recent years, has turned to a casting process known as directional solidification. This technique, disclosed in U.S. Pat. No. 3,260,505, eliminates grain boundaries that lay in a direction transverse to the direction of applied stress in the component. While directional solidification eliminates a major cause of low longitudinal creep-rupture ductility, it is an expensive procedure and is therefore used only in specialized cases where cost is not a major concern.
It has also been attempted to circumvent ductility trough problems by introducing hafnium to nickel-base superalloys (see, e.g., U.S. Pat. Nos. 3,005,705; 3,677,746; 3,677,746; 3,677,747; and 3,677,748). The addition of very dense and expensive hafnium imposes higher raw material costs and increases the unit weight of the alloys. Increased weight, of course, is a serious disadvantage in alloys intended for aircraft engine components. As is apparent, the lack of the combination of high temperature creep-rupture strength and ductility remains a major inadequacy in existing superalloy compositions. These inadequacies are particularly acute since they impair the usefulness of superalloys for many of their intended applications, i.e., formation of gas turbine components.
The alloys of the present invention have improved high temperature strength and corrosion resistance. These alloys are capable of withstanding prolonged operation at temperatures up to about 2,000.degree. F. or higher, and may be formed into highly advantageous castings.
In accordance with the present invention, alloy compositions have been discovered which possess unique and unusually high creep-rupture strength and ductility in the polycrystalline (non-directionally solidified) form. Specifically, a previously unrecognized criticality has been discovered in the amounts of two alloying elements (boron and carbon) included in chromium, aluminum, and titanium containing nickel base superalloy compositions.
The desirability of adding boron and carbon to high temperature alloys is well documented in the prior art technical and patent literature. The alloy characteristics generally enhanced by the addition of some boron and carbon include ductility, strength, forgeability and in some cases, castability. The present level of technology in the field of superalloy physical metallurgy does not enable precise definition or explanation of the exact mechanism responsible for this property enhancement. Yet one versed in the art of superalloy development recognizes the necessity for the presence of both elements.
While it is known that the role of both carbon and boron in nickel superalloys is complex and dynamic, some generalizations can be drawn. Carbon appears in the form of complex carbides which prefer grain boundaries as location sites. Detrimental effects on ductility have been noted with certain grain boundary carbide morphologies. This indicates that carbon should be maintained at low levels. On the other hand, it also has been observed that low carbon content results in sharply reduced high temperature creep life. It is generally believed, since carbides exert a significant and beneficial effect on rupture strength at high temperature, that carbon should be part of superalloy composition.
Boron is considered an essential ingredient in superalloys. In superalloys, boron in the form of complex borides, is also located at grain boundaries. Grain boundary morphology of superalloys is significant because high temperature creep and rupture failures initiate at and propogate along grain boundaries. Complex borides at grain boundaries reduce the onset of grain boundary tearing under rupture loading.
Typical cast superalloys of the prior art preferably contain carbon in an amount of about 0.10% to about 0.25% by weight. In typical prior art wrought alloys, the carbon content range is between about 0.03% and about 0.15% by weight. For example, in a commercial alloy known as INCO 713, the carbon content is kept as low as 0.05% by weight. Boron content in over fifty prior art alloys studies, preferably is held between 0.007% and 0.03% by weight of the composition. The very small amount of boron used in these commercial alloys demonstrates the potency of the element in affecting properties.
The present invention is based, in part, on the discovery of an unusual and unexpected improvement, in both 1,400.degree. F. creep-rupture strength and ductility of gamma prime strengthened nickel-base superalloys, obtained by increasing boron content up to about twenty times the accepted optimum level. Maintenance of the boron content within this critical range of the present invention not only eliminates the problem discussed earlier, relating to the ductility trough present at temperatures between about 1,300.degree. F. and 1,500.degree. F., but results in a marked increase in creep-rupture strength at those temperatures.
It has also been discovered, in accordance with the present invention, that by reducing the carbon content to a critical upper limit below the amount generally employed in superalloys, it is possible to both effect the improvement in 1,400.degree. F. properties and maintain or improve creep-rupture strength and ductility at temperatures around 1,800.degree. F. This aspect of the present invention is important with respect to items such as gas turbine components requiring enhanced properties at both 1,400.degree. F. and 1,800.degree. F.
Among the alloys of the prior art which will exhibit enhanced properties by following the teachings of the present invention are those disclosed in U.S. Pat. Nos. 3,310,399; 3,164,465; 3,061,426; and 3,619,182. While many of the alloy compositions disclosed in these patents are similar to, and generically overlap with, the alloys of the present invention, none of these patents disclosed, nor do corresponding commercial alloy have, the unusual and surprisingly advantageous properties and characteristics of the alloys of the present invention. This is because of the prior art fails to recognize the critical carbon and boron content ranges of the alloys of the present invention. All of the commercial alloys derived from the patents referred to above contain substantially less than the minimum boron content used in the alloys of the present invention. Additionally, while at least some of these patents suggest broad boron content ranges which overlap the boron content range of the present invention, there is no recognition that high temperature properties will maximize in a narrow range within these broadly disclosed ranges.
The alloys of the present invention, which have very good stress rupture life at elevated temperatures, contain required minimum amounts of nickel, chromium, aluminum and titanium. The chromium affords primary corrosion resistance while the remaining components are essential to the formation of the gamma prime intermetallic compound. Ni.sub.3 (Al, Ti), which forms the basic superalloy structure of this invention. The Ni.sub.3 (Al, Ti) precipitate lends to these alloys their required high temperature strength, and titanium is an important element in providing the strength properties of the present alloys at both room temperature and at elevated temperatures. The presence of significant amounts of Ti strengthener in the present alloys renders them significantly different in character from lower temperature alloys such as those of U.S. Pat. No. 3,005,704, which excludes Ti from its alloys.